Had a very similar problem with a lack of bass, with the stock monsoon head unit. After days of searching finally called a dealer and got a wiring diagram for the plugs into the amp. I have an 04 Grand Prix GT, not 100% sure the amp is the same, but it most likely is, maybe this will help
(Smaller Plug)
Pin A/Lt Grn - Right Front Speaker Output +
Pin B/DK Grn - Right Front Speaker Output -
Pin C/Gray - Left Front Speaker Output -
Pin D/Tan - Left Front Speaker Output +
Pin E/ Orange - Battery +
Pin F/Black+White - Ground
Pin G/Yellow - Front Center Speaker Output +
Pin H/Lt Blue - Front Center Speaker Output -
(Larger Plug)
Pin A1/Gray+Black - Amp. Present Signal
Pin A2 - NOT USED
Pin A3/Purple - Radio On Signal
Pin A4/Brown+White - Left Rear Low Level Signal +
Pin A5/Brown - Left Rear Low Level Signal -
Pin A6/Dk Grn - Left Front Low Level Signal -
Pin A7/Tan - Left Front Low Level Signal +
Pin A8/Tan - Amplifier Control
Pin A9/Dk Blue - Right Rear Speaker Output +
Pin A10/Lt Blue - Right Rear Speaker Output -
Pin A11/Brown - Left Rear Speaker Output +
Pin A12/Yellow - Left Rear Speaker Output -
Pin B1/Lt Green+Black - Rear Subwoofer Output -
Pin B2/Dk Blue+White - Rear Subwoofer Output +
Pin B3 NOT USED
Pin B4/Black - Right Rear Low Level Signal -
Pin B5/Dk Blue - Right Rear Low Level Signal +
Pin B6/Lt Green+White - Right Front Low Level Signal +
Pin B7/Lt Green - Right Front Low Level Signal -
Pin B8 NOT USED
Pin B9/Black - Left Front Tweeter Output +
Pin B10/Yellow - Left Front Tweeter Output -
Pin B11/Lt Green - Right Front Tweeter Output +
Pin B12/Purple - Right Front Tweeter Output -
